A resistance of 1980Ω is connected in series with a voltmeter, after which the scale division becomes 100 times larger. Find the resistance of voltmeter. 

Let R be the resistance of voltmeter. Let n be the number of divisions in the voltmeter. The voltage recorded by each division of voltmeter when current i_g flows through it is 

i_gR/n = V         … (i) 

When resistance is connected in series of voltmeter then 

i_g(R+1980)/n = 100V      … (ii) 

Dividing (ii) by (i), we get 

R + 1980 = 100R

or R = 1980/99 = 20 Ω